In a digital video broadcasting (DVB) system, the value of certain transmission parameters must be known in order to correctly demodulate and decode a transport stream (e.g., an MPEG-2 or other digitally encoded video and/or audio transport stream) from a terrestrial broadcast carrier. In some systems, parameters may be encoded in the broadcast carrier, so that a conventional receiver must begin demodulating and decoding a carrier signal before the transport stream can be recovered. Two such systems are the Digital Video Broadcasting-Terrestrial (DVB-T) and Digital Video Broadcasting-Handheld (DVB-H) standards proposed by the European Telecommunications Standards Institute (ETSI), defining baseline transmission systems for digital television broadcasting.
Referring now to FIG. 1, a functional block diagram of an exemplary conventional DVB encoding and modulation system 100 according to the ETSI DVB-T/H standards is shown. The output of an MPEG-2 transport multiplexer 110 is generally encoded and modulated onto a broadcast carrier by DVB system 112. DVB system 112 may process the transport stream through transport multiplex adaptation and randomization (e.g., for energy dispersal) module 120, outer coder (e.g., using a Reed-Solomon code) 121, outer interleaver (e.g., using convolutional interleaving) 122, inner coder (e.g., using a punctured convolutional code) 123, inner interleaver 124, mapper 125, frame adaptation module 126, Orthogonal Frequency Division Multiplexer (OFDM) 128, guard interval inserter 129, and digital-to-analog converter (DAC) 130. Transmission parameters may be inserted by the frame adaptation module in response to transmission parameter signaling (TPS) module 127.
A variety of transmission parameters may affect the encoding and modulation (and thereby affect the subsequent demodulation and decoding) of the transport stream. For example, two modes of operation are defined: a “2K mode” and an “8K mode.” The “2K mode” is suitable for single transmitter operation and for small single frequency networks with limited transmitter distances. The “8K mode” can be used for single transmitter operation and for both small and large single frequency networks.
The transmission parameters may also specify a modulation type. The system supports quadrature phase-shift keying (QPSK) and different levels of quadrature amplitude modulation (QAM) and different inner code rates to be used to trade bit rate versus ruggedness. The system also supports two level hierarchical channel coding and modulation, including uniform and multiresolution constellation. Data carriers in one OFDM frame are generally modulated using QPSK, 16-QAM, 64-QAM, non-uniform 16-QAM, or non-uniform 64-QAM constellations. The proportions of the constellations generally depend on a transmission parameter α, which can take the three values 1, 2, or 4, where α is the minimum distance separating two constellation points carrying different high priority (HP) bit values divided by the minimum distance separating any two constellation points.
In the multi-resolution case, referring again to FIG. 1, splitter 111 may separate the incoming transport stream into two independent MPEG transport streams, referred to as the high-priority and the low-priority stream. These two bitstreams may be mapped onto the signal constellation by the mapper 125 and/or modulator 128.
The inner coder 123 may encode the data using a range of punctured convolutional codes, based on a mother convolutional code of rate 1/2 with 64 states, generally allowing selection of the most appropriate level of error correction for a given service or data rate in either non-hierarchical or hierarchical transmission mode. In addition to the mother code of rate 1/2 the system supports punctured code rates of 2/3, 3/4, 5/6 and 7/8. If two level hierarchical transmission is used, each of the two parallel channel encoders 123 and 134 may have its own independent code rate. The code rate used is generally encoded as a transmission parameter.
In the ETSI DVB-T/H standards, TPS carriers are used for the purpose of signaling parameters related to the transmission scheme (e.g., channel coding, modulation, etc.). The TPS is transmitted in parallel on 17 TPS carriers for the 2K mode and on 68 carriers for the 8K mode. Every TPS carrier in the same symbol conveys the same differentially encoded information bit. Referring now to FIG. 2, the carrier indices for TPS carriers in an OFDM symbol (e.g., a symbol comprising 6,817 carriers in the 8K mode or a symbol comprising 1,705 carriers in the 2K mode) are shown. In addition to the TPS carriers, an OFDM frame generally contains transmitted data and scattered pilot cells and continual pilot carriers.
Referring now to FIG. 3, the structure of a fully decoded TPS data block is shown. The first segment 310 of the TPS data block (bit so) is an initialization bit for a differential binary phase shift keying (2-PSK) modulation. The modulation of the TPS initialization bit is derived from a pseudo-random binary sequence. The second segment 320 of the TPS data block (bits s1-s16) is a synchronization word. The first and third TPS block in each super-frame (e.g., each group of four OFDM frames) have the synchronization word s1-s16=0011010111101110. The second and fourth TPS block have the synchronization word s1-s16=1100101000010001 (e.g., a binary complement of the synchronization words of the first and third TPS blocks). The next segment 330 of the TPS data block (bits s17-s22) is used as a TPS length indicator (binary count) to signal the number of used bits of the TPS. At present this length indicator has the value s17-s22=010111 if cell identification is not supported and the value s17-s22=011111 if the cell identification is supported.
Segments 340 (bits s23-s47) and 350 (bits s48-s53) generally comprise the usable transmission parameters. The bits in segment 340 currently defined, while segment 350 is reserved in the ETSI DVB-T/H standards for future use. Segment 360 of the TPS data block (bits s54-s67 comprises a Bose-Chaudhuri-Hocquenghem (BCH) error correction code (ECC).
Segment 341 (bits s23-s24) designates a frame number within an OFDM super-frame. Segment 342 (bits s25-s26) designate a constellation (e.g., QPSK, 16-QAM, or 64-QAM). Segment 343 (bits s27-s29) specify whether the transmission is hierarchical and, if so, the value of α. Segment 344 (bits s30-s32) specifies the code rate when non-hierarchical channel coding and modulation are used. When hierarchical channel coding and modulation are used segment 344 specifies the code rate for the high priority level of the modulation and segment 355 (bits s33-s35) specifies the code rate for the lower priority level of the modulation. Segment 346 (bits s36-s37) specifies the value of the guard interval, and segment 347 (bits s38-s39) specifies the transmission mode (e.g., 2K mode or 8K mode).
A conventional approach to decoding and demodulating the video stream is to first decode the TPS information before de-interleaving, de-mapping and decoding the received signals. One drawback of this approach that TPS information is distributed over an OFDM frame, where one frame contains 68 OFDM symbols. In the worst case (e.g., when reception begins after a frame has started, thereby requiring a second frame in order to completely decode and verify a block of TPS data), it may take up to 135 symbols to receive a complete OFDM frame. Thus, in some transmission modes, it may take more than 130 milliseconds to receive a complete frame, thereby delaying decoding and demodulation of the received video signal.
Therefore, it is desirable to provide a faster approach for decoding digital video signals with a reasonable degree of reliability. One solution to this problem involves applying an initial set of video transmission parameter values to one or more digital video signal processes, decoding video transmission parameter information from the digital video signal, and updating the initial set of video transmission parameter values with the decoded video transmission parameter information, as described in U.S. patent application Ser. No. 11/731,144 , filed Mar. 30, 2007, the contents of which are incorporated herein by reference.
However, in order to decode the transmission parameter information from the digital video signal, the decoder must determine the position of the parameter information (e.g., to determine which bits in the transmission parameter signal correspond to segments 340 and/or 350 of FIG. 3). In a conventional approach to obtain the parameter information, a decoder first synchronizes to the synchronization word, decodes the TPS information, and then uses the TPS information de-interleaving, de-mapping and decoding the received signals. In order to get the position of the start of a frame, the synchronization words in TPS are often used.
As shown in FIG. 4, there are generally two types of synchronization words which may be embedded in bits s0-s16 of each frame 410-413. The synchronization word SW0 in frames 410 and 412 generally includes the bit values 0011010111101110, and the synchronization word SW1 in frames 411 and 413 generally includes the complementary bit values 1100101000010001. It is possible that parameter values in each frame (e.g., in bits s17-s67) are identical to one or other of the synchronization words. In order to avoid matching to a false synchronization word, a conventional decoder may attempt to match at least two synchronization words of different types to determine the frame boundary.
One drawback of the conventional method is slow channel acquisition and scanning. In a worst case scenario, a decoder may need to receive 151 OFDM symbols (e.g., if the decoder begins receiving at bit s1 of frame 410, then it will have to receive the symbol containing bit s16 of frame 412, and all of the intervening symbols), in order to obtain two complete synchronization words. This may take more than 150 ms in some transmission modes. (e.g., in 8K mode with a guard interval equal to ⅛ of the useful period and an 8 Mhz transmission bandwidth). The duration may be even longer in some configurations (e.g., with guard interval equal to ¼ of the useful duration and a transmission bandwidth of 6 Mhz, the decoder may take approximately 225 ms to receive 151 OFDM symbols).
Therefore, it is also desirable to provide a faster approach for synchronizing with the frame boundaries of transmission parameter blocks to decode digital video signals.